1. Field of the Invention
The present invention relates to an optical waveguide with a spot size changing core structure and its manufacturing method.
2. Description of the Related Art
As the Internet has been developed, optical communication systems have been commercially in practical use. That is, one of the current optical communication systems is a 2.5 Gb/s system which can transmit the equivalent of more than 30,000 telephone lines. As the capacity of information transmission demands increases, high density wavelength multiplexing systems up to 160 wavelength levels have been commercialized.
In the above-mentioned wavelength multiplexing systems, a band combiner for combining a plurality of signal lights having different wavelengths into a single optical fiber and a band splitter for splitting a wavelength multiplexing light from a single optical fiber into a plurality of signal lights having different wavelengths are indispensable. Array waveguide gratings (AWGs) are used as band combiners and band splitters.
Generally, an array waveguide grating is constructed by a plurality of array optical waveguides each having a definite path difference, thus serving as a high-order diffraction grating. A typical array waveguide grating has a size of about 3 cmxc3x974 cm which is encapsulated in an about 5 cmxc3x976 cm package whose temperature is adjusted by a Peltier element. The array waveguide grating is required to be reduced in size, so that the array waveguide grating can be easily mounted on a board.
Note that other optical waveguides used in main lines as well as bidirectional access lines are also required to be reduced in size.
The size of the above-mentioned optical waveguide is dependent upon the curvature radius of the optical waveguides therein. Also, the larger the specific refractive index difference xcex94=(n1xe2x88x92n2)/n1 where n1 is a refractive index of a core layer and n2 is a refractive index of a clad layer, the smaller the loss of light transmission. For example, in an optical waveguide whose xcex94 is about 0.4%, the curvature radius has be larger than 20 mm in order to reduce the loss of light transmission below 0.1 dB/cm. Also, in an optical waveguide whose xcex94 is about 1.2%, the curvature radius can be smaller than 3 mm in order to reduce the loss of light transmission below 0.1 dB/cm. Further, in an optical waveguide whose xcex94 is about 2.0%, the curvature radius has be smaller than 1.5 mm in order to reduce the loss of light transmission below 0.1 dB/cm.
On the other hand, when the difference xcex94 is large, the cross section of a core layer has to be reduced to satisfy the single mode condition of transmission light. As a result, the coupling loss due to the difference in spot size between the facet of an optical waveguide and an optical fiber connected thereto is increased.
Thus, in order to reduce the coupling loss due to the difference in spot size, spot size changing core structures have been suggested.
In a first prior art method for manufacturing an optical waveguide, the width of a core layer is changed while the height of the core layer is definite, to change the spot size near the facet thereof.
In the above-described first prior art method, however, although the confinement of propagation light in the width direction can be weakened, the confinement of propagation light in the thickness direction is not weakened. Therefore, if the propagation light is a circularly-polarized light, the propagation light characteristics deteriorate due to the unbalance of the propagation light in the width direction and the thickness direction.
In a second prior art method for manufacturing an optical waveguide (see: JP-A-2000-206352), a flame hydrolysis deposition (FHD) process is used to form a tapered core layer whose thickness is gradually changed.
In the above-described second prior art method, however, the FHD process is not accurate, so that the configuration of the optical waveguide in the thickness direction can not be accurately controlled.
In a third prior art method for manufacturing an optical waveguide (see: JP-A- 11-84156 (U.S. Pat. No. 6,003,222) and JP-A-2000-137129), a first core layer having a stepped portion, and a second core layer is sintered at the stepped portion of the first core layer, thus obtaining a tapered core layer in the thickness direction.
In the third prior art method, however, since there is a discrepancy between the first and second core layers even when the first and second core layers are made of the same material, propagation light reflects at the interface therebetween and the wave front of the propagation light fluctuates, so that the characteristics of propagation light deteriorate. Also, since the first and second core layers have to be melted at a sintering process, the material of the core layers is limited. For example, a high melting point material such as SiON having a high refractive index cannot be used, and accordingly, the reduction of the optical waveguide in size cannot be expected.
It is an object of the present invention to provide a method for manufacturing an optical waveguide capable of improving the characteristics of propagation light and reducing the size.
According to the present invention, in a method for manufacturing an optical waveguide, a core layer is formed on a clad layer, and a stepped portion is formed in the core layer. Then, a planar layer is formed on the core layer so that the planar layer completely covers the stepped portion of the core layer. Finally, the planar layer and the core layer are etched, so that the planar layer is completely removed and the core layer is converted into a tapered core layer.